1. Field of the Invention
The present invention relates to a radio communication apparatus performing radio communications using a plurality of antennas.
2. Description of the Related Art
One of known cellular systems standardized in 3GPP (3rd Generation Partnership Project) is LTE (Long Term Evolution). In the LTE, SC-FDMA (Single-Carrier Frequency-Division Multiple Access) is adopted as a modulation format of an up link. In addition, in LTE-Advanced (E-UTRA Rel-10 and later), simultaneous communications using a plurality of antennas are often performed in accordance with the class of a mobile station (UE).
A base station (Node B) of LTE (including LTE-Advanced) receives SRS (Sounding Reference Signal) including known signals transmitted from the mobile station and evaluates a channel characteristic (Channel state) of the up link of the mobile station to obtain transmission path characteristic CSI (Channel State Information). See, for example, document 3GPP TS 36.211 V8.6.0 (2009-03) 5.5.3 Sounding reference signal.
The base station generates schedule information allocated by a resource block for the mobile station, on the basis of the obtained CSI, and selects Index of Pre-coding codebook which defines a transmission method of MIMO and MCS Index which defines modulation and coding, and transmits the information to the mobile station over a control channel (for example, PDCCH) of the down link. The mobile station selects the resource block, MCS, Pre-coding codebook on the basis of the information received over the control channel to perform communications over a channel (for example, PUSCH) of the up link.
Incidentally, the SRS is multiplexed at the same time (sub-frame) in the same frequency band (resource block) and then transmitted. More specifically, the SRS is subjected to stagger arrangement in a frequency direction so as to be orthogonalized with frequency or orthogonalized with code by using different sequences between mobile stations.
The stagger arrangement merely includes an even-number arrangement of arranging signals in even-numbered resource blocks and an odd-number arrangement of arranging signals in odd-numbered resource blocks. In addition, since the frequency band needs to be limited to a band in which frequency correlation can be maintained, in response to a transmission path variation in the frequency direction, an orthogonal sequence length of the SRS is limited. The SRS used in a wide band is implemented by connecting a plurality of short orthogonal sequences. In addition, increasing the number of orthogonal sequences by performing cyclic shift for the sequences is also executed.
In a system of performing simultaneous transmission using a plurality of antennas, however, transmitting signals by combining SRS orthogonal in frequency or code between the antennas is desirable for CSI measurement, but a problem arises that combinations of orthogonalization in frequency or code of the SRS are short as explained above.
When signals are transmitted by using a plurality of in a radio communication system of antennas performing evaluation of transmission path characteristics by transmitting the known signals, use of the known signals orthogonal in frequency or code between the antennas is desirable for the evaluation of the transmission path characteristics, but a problem arises that the combinations of orthogonalization in frequency or code are limited.